1. Field of the Invention
The present invention relates to a dry etching method and apparatus for manufacturing a semiconductor device, and more particularly, to a dry etching apparatus for enhancing the uniformity of etching a wafer used to manufacture a semiconductor device.
2. Description of the Related Art
As semiconductor devices become more highly integrated, more precise wafer engineering skills and methods are needed to manufacture a very highly-integrated semiconductor device. Accordingly, the etching process, which is an important semiconductor manufacturing process, must become more precise.
Efficient etching generally requires variable selection, a high etching rate, uniformity, stability, and resultant low damage to the semiconductor substrate. Also, precise etching involves accurately controlling the etching rate according to the way in which a wafer to be processed is situated.
Known etching processes are roughly divided into dry etching and wet etching, which are selectively employed because the characteristics of each process have their own particular advantages and disadvantages. Wet etching is widely used and has advantages of low cost, high selectivity, high etching rate, and high reliability. However, wet etching can not be used to precisely etch a narrow line width and it is subject to the problem of undercutting since the chemical etching process is isotropic.
Dry etching encompasses several different types of processes, including: physical etching, such as Ion Milling; physical and chemical etching such as Reactive Ion Etching (RIE); and chemical etching, such as plasma etching. Physical etching results in a more precise pattern transfer as it is an anisotropic process, but it has a low selectivity rate. On the contrary, high selectivity is a characteristic of plasma etching, but plasma etching is also an isotropic process subject to the problem of undercutting.
Many factors contribute to the quality and quantity of etching in a plasma etching process, including the types of cooling gas, the temperature and pressure of the processing chamber, the distribution and density of the plasma particles of the cooling gas, the energy level of the plasma and the like. These factors closely correlate with the energy applied to the etching equipment and its structure.
Etching equipment is divided into two categories; batch-type equipment, which processes a plurality of wafers at one time, and single-type equipment, which processes wafers one by one. Conventionally, a batch-type etching process is widely used for small-diameter wafers because the non-uniformity of etching within a wafer or between wafers is not a big problem for the small-diameter wafers.
On the other hand, the non-uniformity of etching within or between wafers having larger diameters, e.g., 8 inches or 12 inches, is a major concern. Therefore, single-type etching equipment is preferably used to etch such large wafers in order to provide more uniformly etched wafers.
However, the uniformity of the etching process is still problematic due to many other operational parameters of the equipment. This problem is especially serious when manufacturing highly-integrated semiconductor devices. Some of the factors having an impact on the etching uniformity are described as follows, referring to the attached drawings.
FIG. 1 is a schematic view of an embodiment of a conventional etching apparatus, wherein the apparatus comprises a susceptor 12 supporting a wafer 13 inside a processing chamber 10, a high frequency voltage power source 16 for the susceptor, and an electrode 11 facing the susceptor 12.
The susceptor 12 functions as a lower electrode. The RF power source 16 is connected to the susceptor 12 through a condenser to supply high frequency power thereto. In addition, helium is supplied to the backside of susceptor 12 through helium supply tubes 14 and 15. The former tube 14 is used for improving the etching uniformity by maintaining the temperature of the wafer at a uniform level, and the latter tube 15 is used for cooling the susceptor 12. The upper electrode 11 facing the susceptor 12 is usually made of a metal plate or coil and is grounded. Alternatively, the upper electrode 11 may have a high frequency power source connected thereto whereas the susceptor 12 is grounded. The upper electrode 11 includes a gas diffuser having a plurality of nozzle openings and an induction coil, both of which are integral parts of the upper electrode.
Whereas the upper electrode 11 has a flat shape, the lower electrode 12 has a convex shape in most cases due to the helium supplied to the wafer. The helium, which is used for improving the uniformity of the etching process, is diffused through tubes 14 onto the center of the backside of the wafer 13 at a pressure that is slightly higher than the pressure inside the processing chamber 10. Consequently, the wafer 13 is flexed convexly, or upward as shown in FIG. 1.
Also, the applied high frequency electric field is more intense at the middle of the wafer 13 than at its periphery such that the plasma particles become more highly concentrated at its middle even with the uniform cooling gas being supplied. Therefore, it is difficult to obtain uniformity in the etching process as well as maintaining the etching rate at a desired level.
FIG. 2 shows the distribution density of a plurality of nozzle openings of the conventional gas diffuser of the upper electrode. The round-plate shaped gas diffuser 21 has a plurality of gas nozzle openings 22 that are uniformly distributed over the surface of the gas diffuser 21.
The density of source gas is always slightly less at the edge of the wafer because the by-product reaction gas is discharged from the periphery of the wafer to the side-wall of the processing chamber. Therefore, it is usually necessary to increase the density of the source gas at the edge of the wafer to compensate for this.
FIG. 3 is another schematic view of the RF power source applied to the susceptor 12 on which a wafer 13 is mounted in a processing chamber (not shown). The RF power is a significant factor affecting the etching rate, but it is difficult to partially control the applied RF electric field in the etching apparatus as shown in FIG. 3. Furthermore, the etching rate varies within the wafer 13 according to the temperature differences therewithin. These temperature differences are difficult to control and compensate for using the conventional etching apparatus because the backside cooling helium is supplied in a uniform, continuous manner at a constant density.
Accordingly, these factors affecting the etching rate should be controlled to maintain the density of the activated plasma along the wafer surface at a constant level.